Integrated circuits are formed on a semiconductor die and packaged for incorporation into a variety of end products. Packaging integrated circuits typically includes placing a die on a substrate and forming electrical connections between input-output (I/O) pads on the die and conductive traces on the substrate.
Flip-chip package is a well-known type of integrated circuit package, comprising a flip chip coupled to a substrate through an array of conductive bumps and an underfill material that fills the gap between the chip and the substrate and encapsulates the conductive bumps. An array of solder balls are provided on another surface of the substrate and serves as input/output connections for the package. In order to prevent warpage, the substrate is typically a thick substrate, with a thickness of about 1.2 mm with a 0.8 mm core.
In recent years, thin core and coreless substrates have emerged to address the high electrical performance required for advanced electronic products. Thus, thin core substrate of 0.4 mm or 0.2 mm, and also coreless substrates have been used. Such substrates improve the electrical performance of a package by reducing the lengths and distances of circuit paths and electrical connections.
However, thin core or coreless substrates are subject to warpage during the assembly and test manufacturing process, leading to poor contacts between the substrate and the conductive bumps, or cracks of the conductive bumps, thereby degrading the electrical contact and the product quality. Thus, to enable a package to survive such handling, as well as other stresses in the use environment of the package, stiffeners often are attached to substrates.
FIGS. 1A and 1B (Prior Art) illustrate a perspective view and a cross section view, respectively, of a prior art package stiffening by attaching a metal ring on a peripheral portion of the substrate. The package includes a semiconductor die 11 attached to a substrate 13 through an array of conductive bumps and secured with an underfill 14. Capacitors 12 can also be distributed around the semiconductor die, for example, to store energy to accommodate electrical power ramping. A metal stiffener 10, typically comprising a metal piece with a cutout in the middle, is glued to the substrate 13 to provide stability to substrate 13 by reducing the forces applied to the substrate during handling.
However, metal stiffener can be cost ineffective by occupying surface area of the substrate and thus limiting the available space for various active and passive components. Further, reliability of metal stiffener packages can be affected by the mismatch in thermal expansion coefficient (CTE) between the metal stiffener and the substrate, leading to delamination or broken electrical connections.
FIGS. 2A and 2B (Prior Art) illustrate a perspective view and a cross section view, respectively, of a prior art package stiffened by molding such as transfer molding or compression molding. The package includes a semiconductor die 11 attached to a substrate 13 through an underfill 14, together with capacitors 12. A mold may be placed to enclose the die/substrate package and a molding compound is injected. After curing, the mold is removed and molding compound is solidified to form a mold structure 20 which is adhesively bonded to the die 11 and the substrate 13, providing the necessary volume to maintain a sufficient rigidity to the molded flip chip package.
FIGS. 3A and 3B (Prior Art) illustrate a top view and a cross section view, respectively, of a variation of the prior art package stiffened by molding. A mold is placed on an empty substrate 13 (e.g., substrate before attaching the semiconductor die 11) and a molding compound is injected to form an encapsulant mold structure 30 after appropriate thermal curing. Die 11 and underfill 34 can be provided to the substrate 13 after the formation of the encapsulant mold structure 30.